Sound shielding by means of sound

ABSTRACT

An air permeable sound grating for sound shielding. The grating consists of, or includes, volume changing elements which are controlled by properly positioned means, such as a microphone orientated toward the noise source, to effect change in volume of an adjacent noise transmitting medium, such as air, to effect such a anticyclic change in volume as to cause interference attenuation of the sound at the side of the grating facing away from the noise source. Such volume changing elements are spaced apart a distance less than the wavelength of the highest frequency sound to be shielded and may comprise a variety of means such as spaced heating wires, conductive coating upon a thermal and electrically insulating base, a closed capsule with membrane surfaces having vibration generating means operatively associated with said membrane, radiation generating means for discharging medium heating radiation of energy into the sound transmitting medium together with controls and reflectors for directing the volume changing means or energy to the desired zone to be shielded.

United States Patent [72] Inventor Oslrnr Bschorr Munich, Germany [21 Appl. No. 27,429 221 Filed Apr. 10, 1970 [4S] Patented Aug. 31, 1971 [73] Assignee Messerschmltt-Bolkow-Blohm GmbH Munich, Germany [32] Priority Apr. 12, 1969 11 Germany [31] P 19 18 741.8

I 54] SOUND SHIELDING BY MEANS OF SOUND 20 Claims, No Drawings [52] US. Cl 181/33 L, 181/64 B,179/1P 51] m. c| H04m 1/19, FOln 1/06 [50] Field otSearch 181/33, 33 D, 33 L, 35, 41, 49, 64, 64 B; 179/1 8 [56] References Cited UNITED STATES PATENTS 3,;51648 8/1951 Hammond 177/1.8 *3508 7/1959 Baruch et al.... 181/64 (.2) $883,790 5/1961 Olson 181/33 (.5)

FOREIGN PATENTS 1,098,730 2/1961 Germany Primary ExaminerRobert S. Ward, .lr. AuorneyWoodhams, Blanchard and Flynn ing elements which are controlled by properly positioned means, such as a microphone orientated toward the noise source, to effect change in volume of an adjacent noise transmitting medium, such as air, to effect such a anticyclic change in volume as to cause interference-attenuation of the sound at the side of the grating facing away from the noise source. Such volume changing elements are spaced apart a distance less than the wavelength of the highest frequency sound to be shielded and may comprise a variety of means such as spaced heating wires, conductive coating upon a thermal and electrically insulating base, a closed capsule with membrane surfaces having vibration generating means operatively associated with said membrane, radiation generating means for discharging medium heating radiation of energy into the sound transmitting medium together with controls and reflectors for directing the volume changing means or energy to the desired zone to be shielded.

SOUND SHIELDING BY MEANS OF SOUND The present invention relates to sound gratings, permeable to air, the sound shielding of which is based upon the principle of interference attenuated with an antinoise field and which is to be used where sound attenuation by solid walls is not possible. These sound gratings can be used as emission and as admission protection.

Sound shielding effect of an antinoise field is based upon 'superimposing an anticycle sound field upon a noise field which produces interference attenuation. In order to attenuate a noise field X, it is necessary that X'(p. I)= (p. (X =sound state vector, p sound pressure, v particle velocity vector, r= site vector, t= time) It is the object of the invention to provide netgrating-or fencelike devices, called here sound gratings, which assure a complete, partial or selective sound shielding. A further object, especially for large area sound shielding, is to provide nonmechanical sound shielding based on light.

The fundamental principle of the sound grating is to produce, by means of the grating, or with elements attached thereto, a controlled, volume-changing movement. Volume changing such as this produces sound. This propagation of sound is oriented by a directional microphone toward the noise source to be shielded and is controlled in accordance with the principle of an antinoise field.

In this way, the noise field and the antinoise field are attenuated at the side facing away from the noise source. The sound grating acts in this case as if it consisted of a solid, reflecting wall.

In the form just described, the sound gratingattenuates only sound originating on the microphone side, while sound from the other side can pass through freely. If a directional microphone is also attached at this side, and its signals are used to control the antinoise probe in the sound grating, the latter then becomes impermeable to sound from both sides. It is possible to switch on at will the blocking directions one at a time or simultaneously.

Conditions are most simple when the noise waves to be shielded strike the sound grating at right angles. In this case, the volume-changing elements and the antinoise probes work in phase and can essentially be controlled by one microphone. If the sound sources come from various directions and large area sound gratings are used, several microphones are needed, each of which controls a section of the sound grating. A further improvement results when the control sections of the various microphones overlap so that one antinoise probe is controlled by several microphones in accordance with the distance to the microphone. If two sound gratings, permeable from one side are arranged so that the impermeable sides face each other, the arrangement then works like a trap where the sound can enter the space between the two gratings, but cannot leave it. If, for example, sound is propagated upwards, the device works like an absorbent wall.

A sound grating can, depending upon the way in which it is used, be modified so as to perform an additional function, for example, as a mechanical barrier, a self-supporting fence or as a decorative gating.

One illustrative embodiment, in order to provide admission protection, for example, at a window opening has electrically conducting wires'suspended across the opening. The spacing between wires should be less than the wavelength of the sound to be shielded. If this system of wires is heated with electric current, an increase in volume occurs due to heating of the air. If the wire is heated with alternating current, the alternating air expansion produces sound. As the heating effect increases as the square of the current, the frequency of the sound emitted is twice that of the current.

However, thereis a definite relationship between the current and the resultant acoustic efi'ect. By controlling the amount of current, it is therefore possible to produce a required antinoise field. In order to reduce the loss of heat, and in order to achieve the highest possible frequency limit, the heat capacity, i.e., the wire diameter, must be as small as possible. In one desirable embodiment the fine heating wires are best woven into a protective net in order to provide mechanical protection against tensile and contactdamage. It is also possible to coil the heating wires and to house the coils in a bell which is open toward the bottom. The individual bells are located, for example, at the junctions of a support grating.

Instead of wire heating, surface heating can also be used. In this case, an extremely thin metal layer is applied to electric and thermal insulating surface pieces. This metal layer is heated either directly or by induction through a coil in the surface. The individual surfaces are located at the junctions of a support grating through which the current is also supplied.

In another embodiment, capsules closed by membranes are attached at the junctions of a support grating. The membranes, as in an electric loudspeaker, can be moved so that the volume of the capsule is changed. This capsule system can very easily be controlled electrodynamically, so that a given antinoise field can be; verified. It is wise to have the individual frequency of the membrane the same as the maximum frequency of the noise source.

In addition to electricity, hydraulic power can also be used as the energy for an antinoise field. One possibility is to expose a mesh made of elastic hose filled with hydraulic fluid to a changing hydraulic pressure. In this way, the volume of the hose changes and sound is emitted in the known way. In place of elastic hoses, it is also possible to provide at the junctions of a support grating hydraulically filled elastic capsules or pillows. The required antinoise field can be controlled with hydraulic pressure. A nonmechanical noise grating can be produced by using light. Light should be understood to include any radiation of energy-w'ave radiation or particle radiation-the propagation velocity of which is much greater than that of sound. In addition, this radiation must be absorbed by the air such that the air is heated or that a volume-changing chemical reaction occurs. A light beam or a planar light fan can, when suitably controlled, be used as an antinoise generator and works like a sound grating. In one illustrative embodiment, a central source of light is used, radiating essentially in the form of a disc or, to compensate a curved sound wave front, in the form of the surface of a low cone. The light intensity is controlled by known means through the signal from" a directional microphone like an antinoise generator. The complete system is capable of swiveling and is oriented and moved by a mimic directed toward the noise source.

If, from a supersonic aircraft, the low-pressure wave of a sonic boom is heated with such a light source, the low-pressure wave can be attenuated. Due to power considerations, only the part radiated downward is attenuated, as the upper part does not cause any disturbance.

In another illustrative embodiment, a linear-shaped light source is used instead of a central source and the light is radiated in a shielding plane. Through control of the intensity, as in an antinoise generator, a noise grating is produced. If the sound shielding is used on a small area, for example, at a window opening, it is desirable to use reflectors at the edges which cause the light to be reflected several times so as to provide better use of the energy.

By providing a closed noise grating, it is possible to produce, a sound cage that is impervious to sound.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Sound shielding with permeable areas characterized in that a net-, gratingor fencelike noise grating is used, consisting of volume-changing elements, the volume change of which is controlled by a directional microphone oriented toward the noise source, the control being such that the resultant sound field along with the noise field causes interference attenuation at the side of the grating facing away from the noise source.

2. Sound shielding with permeable areas in accordance with claim 1, having the spacing between the volume-changing elements smaller than the wavelength of the highest frequency sound to be shielded.

3. Sound shielding with permeable areas in accordance with claim 1 having, to achieve sound shielding at both sides, a

ble sides are facing each other at a certain distance apart.

6. Sound shielding with permeable areas in accordance with claim 1 having in the case of large area sound shielding, several directional microphones, each controlling only a section of the sound grating.

7. Sound shielding with permeable areas in accordance with claim 1 having the control sections controlled by the individual microphones overlapping and having the volumechanging element controlled by several microphones in accordance with the distance to the microphone.

8. Sound shielding with permeable areas in accordance with claim 1 having several noise gratings assembled together so as to form a closed or partially open sound cage.

9. Sound shielding with permeable areas in accordance with claim 1, the noise grating serving simultaneously as a mechanical barrier, as a self-supportingfence or as a decorative grating wall.

10. Noise grating in accordance with claim 1 where the volume-changing elements are electrically conducting wires energized by a frame, the heating power being controlled by control of the current.

11. Noise grating in accordance with claim 1 having as volume-changing elements coiled heating wires contained in an open bell, the bells being located on a support grating.

12. Noise grating in accordance with claim 1 having as a volume-changing element a thin metal coating upon a thermal and electrical insulating surface piece, the heating power of this element being controlled by current control and the surface piece being located on a support grating.

13. Noise grating in accordance with claim 1 havingas volume-changing elements closed capsules with membrane surfaces, the latter being controlled by a coil/magnet system located inside the capsule.

14. Noise grating in accordance with claim 1 having as a volume-changing element an elastic hose mesh, balls or pillows, which are filled with hydraulic fluid, the volume of such elements being controlled by hydraulic pressure.

15. Noise grating in accordance with claim 1 having as a volume-changing element a radiation absorbed by air, the velocity of the radiation being much greater than the velocity of sound and the intensity being controlled like an antinoise generator.

16. Noise grating in accordance with claim 15 having the radiation of energy in the form of a disc or the surface of a flat cone.

17. Noise grating in accordance with claim 16 having the orientation of the disc or conical surface of energy radiation controlled so that the sound wave to be attenuated strikes the disc surface at right angles.

18. Noise grating in accordance with claim 17 where the conically shaped low pressure wave of the sonic boom is heated by a radiation of energy from a supersonic aircraft, the radiation being capable of being absorbed by air.

19. Noise grating in accordance with claim 1, the volumechanging element being a line-shaped source of energy, the

energy being radiated in a plane.

20. Noise grating in accordance with claim 1 having, in the case of sound shielding on a limited area, reflectors at the outer borders which cause multiple reflection of the energy radiation. 

1. Sound shielding with permeable areas characterized in that a net-, grating-or fencelike noise grating is used, consisting of volume-changing elements, the volume change of which is controlled by a directional microphone oriented toward the noise source, the control being such that the resultant sound field along with the noise field causes interference attenuation at the side of the grating facing away from the noise source.
 2. Sound shielding with permeable areas in accordance with claim 1, having the spacing between the volume-changing elements smaller than the wavelength of the highest frequency sound to be shielded.
 3. Sound shielding with permeable areas in accordance with claim 1 having, to achieve sound shielding at both sides, a directional microphone at each side of the noise grating, the signals of which are superimposed, the overall signal controlling the volume-changing elements.
 4. Sound shielding with permeable areas in accordance with claim 1 having a switching capability so that shielding may be had at either side or at both sides simultaneously.
 5. Sound shielding with permeable areas in accordance with claim 1 having, for the purpose of creating an absorbent wall, two noise gratings, each working only from one side, so arranged with respect to each other that their sound-impermeable sides are facing each other at a certain distance apart.
 6. Sound shielding with permeable areas in accordance with claim 1 having in the case of large area sound shielding, several directional microphones, each controlling only a section of the sound grating.
 7. Sound shielding with permeable areas in accordance with claim 1 having the control sections controlled by the individual microphones overlapping and having the volume-changing element controlled by several microphones in accordance with the distance to the microphone.
 8. Sound shielding with permeable areas in accordance with claim 1 having several noise gratings assembled together so as to form a closed or partially open sound cage.
 9. Sound shielding with permeable areas in accordance with claim 1, the noise grating serving simultaneously as a mechanical barrier, as a self-supporting fence or as a decorative grating wall.
 10. Noise grating in accordance with claim 1 where the volume-changing elements are electrically conducting wires energized by a frame, the heating power being controlled by control of The current.
 11. Noise grating in accordance with claim 1 having as volume-changing elements coiled heating wires contained in an open bell, the bells being located on a support grating.
 12. Noise grating in accordance with claim 1 having as a volume-changing element a thin metal coating upon a thermal and electrical insulating surface piece, the heating power of this element being controlled by current control and the surface piece being located on a support grating.
 13. Noise grating in accordance with claim 1 having as volume-changing elements closed capsules with membrane surfaces, the latter being controlled by a coil/magnet system located inside the capsule.
 14. Noise grating in accordance with claim 1 having as a volume-changing element an elastic hose mesh, balls or pillows, which are filled with hydraulic fluid, the volume of such elements being controlled by hydraulic pressure.
 15. Noise grating in accordance with claim 1 having as a volume-changing element a radiation absorbed by air, the velocity of the radiation being much greater than the velocity of sound and the intensity being controlled like an antinoise generator.
 16. Noise grating in accordance with claim 15 having the radiation of energy in the form of a disc or the surface of a flat cone.
 17. Noise grating in accordance with claim 16 having the orientation of the disc or conical surface of energy radiation controlled so that the sound wave to be attenuated strikes the disc surface at right angles.
 18. Noise grating in accordance with claim 17 where the conically shaped low pressure wave of the sonic boom is heated by a radiation of energy from a supersonic aircraft, the radiation being capable of being absorbed by air.
 19. Noise grating in accordance with claim 1, the volume-changing element being a line-shaped source of energy, the energy being radiated in a plane.
 20. Noise grating in accordance with claim 1 having, in the case of sound shielding on a limited area, reflectors at the outer borders which cause multiple reflection of the energy radiation. 